It is known that the preparation of urea from ammonia and carbon dioxide is in two steps, via the intermediate product ammonium carbamate; under the usual conditions of temperature and pressure the conversion of ammonia and carbon dioxide into ammonium carbamate according to the exothermic reaction: EQU 2 NH.sub.3 + CO.sub.2 .revreaction. NH.sub.2 COONH.sub.4 + a kcal
Is practically complete, but the conversion of the ammonium carbamate into urea according to the endothermic reaction EQU NH.sub.2 COONH.sub.4 .revreaction. CO(NH.sub.2).sub.2 + H.sub.2 O - b kcal
Is only a partial one.
Synthesis pressures used in modern processes for preparation of urea are between 125 and 200 atm. and the synthesis temperatures used are between 180.degree. and 200.degree.C. These temperatures are in agreement with the findings of E. Otsuka, who investigated the conversion of ammonia and carbon dioxide into urea at different temperatures and molar ratios of H.sub.2 O to CO.sub.2 and found for each H.sub.2 O to CO.sub.2 ratio that the equilibrium conversion passes through a maximum at approximately 190.degree.C. See Hydrocarbon Processing, June 1970, pages 111-115.
In order to increase the degree of conversion according to another known expedient, the urea synthesis is carried out in the presence of an excess quantity of ammonia. The solution obtained during the synthesis therefore contains, in addition to urea and water, non-converted ammonium carbamate and free NH.sub.3. The ammonium carbamate and free ammonia should be removed from the solution before the urea solution can be concentrated and be processed into a salable end product.
According to the process described in U.S. Pat. No. 3,356,723, the non-converted ammonium carbamate and free NH.sub.3 are removed by subjecting the urea synthesis solution, under pressure, to a countercurrent stripping treatment with gaseous carbon dioxide, in which the ammonium carbamate is decomposed into NH.sub.3 and CO.sub.2, with expulsion of a gaseous mixture of NH.sub.3, CO.sub.2 and H.sub.2 O. This gas mixture is condensed and returned as an ammonium carbamate solution to the urea synthesis zone. The disclosure of U.S. Pat. No. 3,356,723 in the name of Kaasenbrood, one of the coinventors herein, is hereby incorporated by reference to the extent necessary to fully describe the present invention.
The decomposition of the ammonium carbamate and the expulsion of the ammonia and the carbon dioxide are effected in the stripping process by reduction of the partial pressure of one of the two reaction components with the aid of the stripping gas and with the addition of heat. The decomposition can be hastened by using a pressure as low as possible at which the stripping treatment takes place. However, condensation of the gases discharged from the stripping zone should take place at as high a pressure as possible in order to assure the ammonium carbamate formation at as high a temperature as possible, so that there are more possibilities of applying the liberated heat and, also, that the ammonium carbamate is returned to the urea synthesis zone with as little water as possible -- a desirable factor in view of the detrimental influence of water on the conversion to urea. For these reasons the condensation is carried out at the pressure at which the urea synthesis takes place so that there is only a small pressure differential, if any, between the condenser and the urea synthesis zone.
The gas mixture discharged from the stripping zone, which as mentioned, contains water in addition to ammonia and carbon dioxide, can only be compressed if special measures are taken to avoid formation of liquid or solid phases in the compressor and in the lines. Moreover, a particularly high amount of energy is needed for driving the compressors. In practice, therefore, the stripping treatment is carried out at the same pressure as the condensation and the synthesis. Actually, this pressure is determined by the maximum temperature level at which the heat required for the stripping treatment can still be supplied without hydrolysis of the urea formed and formation of biuret occurring to an unacceptable degree.
Further, to minimize or avoid hydrolysis and formation of biuret in the stripping zone, it is essential that the residence time of the urea synthesis solution in the stripping zone be as short as possible. This is achieved by passing the solution along a wall in the form of a fast flowing film; the heat required is applied external to the wall and to accomplish this a vertical tubular heat exchanger is used in practice, through the shell or external side of which a heating medium flows, while the urea synthesis solution flows along the internal walls of the tubes in a direction counter-current to the ascending stripping gas. In order to avoid excess local heating of the urea synthesis solution and the hydrolysis and subsequent formation of biuret therefrom, it is necessary that the liquid to be stripped be applied as evenly as possible to the internal walls of the tubes. In view of this requirement, high demands are made on the distribution of the liquid over the interior of the tubes and one must use accurately dimensioned distributors. Further, care should also be taken that the stripping gas is evenly distributed over the internal areas of the tubes, which can be accomplished by providing the gas discharge ends of the tubes with identical orifices.